The Navy has been increasingly involved in salvage operations. These operations include the recovery of various hardware placed on the ocean bottom as well as objects which are inadvertently dropped to the ocean bottom. The Navy has a continuing program to improve its capability and equipments for recovering submerged objects.
Salvage operations generally utilize lifting lines buoyant lift structures, or a combination of these methods. The use of buoyant lift structures, either rigid or collapsible, requires placement, attachment, and either deballasting or inflation of the buoyant body. The requirements for salvage operations at deep ocean depths have become more frequent, consequently requiring an advance in the state of the art of the equipment.
The primary problem in using buoyant recovery systems is controlling the ascent velocity once the lift has begun. This is particularly true with collapsible lift bags. If the load to be lifted is less than the bag capacity, ascent will begin before the bag has reached its maximum displacement. If the bag is closed, it will normally have folds which decrease its volume below the fully inflated volume, and if it is open, the bag will normally have both folds and water inside. In either case, as the bag ascends, the gas expands forcing out the water and/or the folds, thereby increasing the net buoyancy of the bag. As the buoyancy increases, the ascent velocity increases which can result in a very hazardous uncontrolled ascent of the object to be salvaged. Further, during a rapid ascent, a velocity head will develop at a stagnation point at the top of the bag. When this velocity head exceeds the internal pressure opposite the stagnation point the top of the bag will deform. This leads to an unstable bubble geometry at the top of the bag and will normally result in substantial horizontal excursions of the lifting bag.
Collapsible bags are often preferred to rigid pontoon type lifting devices because the former can be easily stored and transported when not in use. However, the hazardous ascent velocities of the collapsible bags have been a deterrence, which problem the present invention seeks to overcome.